Electro-Harmonix Mini-Synthesizer

Aug 31, 2016

The Electro-Harmonix EH-0400 Mini-Synthesizer: A surprising little synth with a big sound.

Recently Craig bought an EH Mini-Synthesiser and after we gave it a service and check out we had a good play with it through a studio monitor system. We were surprised at just how good this little box can sound. It is limited in what it can do, it’s little touch keyboard is somewhat squirrelly and the bottom panel is made of cardboard but it has a huge bottom end and it’s dual filter and odd control layout, it’s quirks and limitations actually add up to some really unique sounds that other mono synths just can’t duplicate. It is one of many examples of the wonderful synthesizer legacy of designer David Cockerell.

Elecro-Harmonix has released a Mini-Synthesizer iPad app so it seemed like a good time to take a look inside the real thing to see if we can discover why it sounds like it does. It’s a few seconds work to remove the bottom panel to expose the insides, here’s what we found… As always, click on the photos for a closer view.

The retro and very cool front panel.

The little touch keyboard, enough said.

Once we got inside a few things are obvious – it has a little speaker, a couple of batteries and a lot of hand soldering. The first noticeable thing is the jumble of resistors soldered along the keyboard resistor chain. As the resistors EH used are standard components with standard values there must have been a lot of experimenting with combinations of resistors in series and parallel to get the correct values between each key to make a chromatic scale. Hopefully the formula worked for each unit, maybe the unpleasant job that they gave the new kid at the EH factory was to sit at a bench with a guitar tuner soldering resistors into Mini-Synthesisers until the keyboard intervals were close enough to put the synth in a box and ship it out. As you can probably imagine the touch keyboard is very basic and is prone to double triggering and clicking when you lift your finger from the key.

The keyboard resistor chain

The keyboard layers exposed

There is a kind of velocity sensitivity – A piezo sensor mounted beneath the keyboard senses impacts to the keys (or anywhere else) and it opens the filter if the key is tapped harder. The synth runs on 9V, there are two batteries inside and unlike many pedals this is not to provide a dual +/- supply, they are actually in parallel in order to provide additional current so that the synth can run for longer without losing tuning as the battery runs down. Because of this it will run on one battery if that’s all you have on hand. When run off a power adaptor the synth runs on around 12V.

The impact sensor – Yes, it has touch sensitivity. Sort of.

The little internal speaker

There isn’t a great deal of technical information available for the Mini-Synthesizer and as you can see from the photos below the markings of the IC’s have been scrubbed off. This was occasionally done by manufacturers in those days to make it more difficult for other companies and hobbyists to copy their circuit design. Looking at the circuit board you can see that there aren’t many components, just 5 IC’s, 5 transistors, a few diodes and some resistors and capacitors. Considering the minimalist design the Mini-Synthesiser manages to be surprisingly playable and can pull some very useable sounds. Sonically it is definitely not a toy, it is a potent, if limited little synth.

The main circuit board

Another view of the internals showing both boards

The sliders and switches are quite solid, all in all it’s a pretty decent effort at making a useable little synth at a price that allowed an entry into synthesis for people who might have wanted some basic synth sounds on stage or for a guitarist to add to their home writing rig but who didn’t want to pay for a full size mono synth. It’s definitely not a lead synth, it is happiest playing in the lower registers and it’s forte is deep filter/pulse width up or down sweeps that have a distinctive moving phase shift character across the sweep range. It’s something of a one trick pony but it does that trick surprisingly well.

A scrubbed IC – Keeping trade secrets

Another scrubbed IC, this time an op-amp

As previously mentioned the service info is a little sketchy but it looks like the basic circuit is a CD4013 dual flip flop IC used as a pulse wave oscillator which also divides the pulse wave to generate the sub octave, a dual low-pass filter based on a pair of CA3094 OTA op-amps, an LM339 quad comparator IC handles keyboard, pitch bend and octave voltages and the pulse width/phase sweep. A TBA820M 1.2W amplifier IC drives the little internal speaker. It uses a combination of pulse width modulation and offsets between the two filters from the 2x switch as they sweep to create a phase shifting effect.

Should you buy one? If you find a Mini-Synthesizer at a decent price and you like it’s classic styling and coolness factor (and you don’t mind it’s fragility) then it’s worth considering and apart from the touch keyboard it has a high repairability score. The iPad app sounds really close though and it has none of the annoying quirks from the touch keyboard. If you are just looking for the Mini-Synthesizer sound the app is realistically the better choice (which is kind of an odd thing to say for a blog like this) but the keyboard on the hardware machine makes smooth playing difficult and pitch scaling is not terribly accurate. If you ever get your hands on the original it’s worth a play, they are a lot of fun.